A power supply apparatus converts electric power obtained from a solar cell, a fuel cell, and a storage battery into AC power which is in a format usable by general households. In Japan, a voltage of the AC power is 100 V, and a frequency is 50 Hz or 60 Hz. Normally, the AC power is supplied to the general households by a commercial power supply grid (hereinafter, abbreviated as a “grid” as appropriate).
The power supply apparatus may be connected in parallel with the grid to carry out a linkage operation therewith. Thereby, to a grid load connected to the grid (such as electrical products of the general households), the power from the solar cell and the power from the grid may be supplied. Also, in case of grid power failure, the power supply apparatuses may be disconnected from the grid and operate independently.
In connecting the power supply apparatus to the grid, either a single or a plurality of power supply apparatuses may be connected in parallel with the grid. For example, a solar cell power supply apparatus and a fuel cell power supply apparatus may be connected in parallel with the grid to carry out the linkage operation therewith.
In using the power supply apparatus to carry out the linkage operation with the grid, from the viewpoint of protection of the grid, it is important to prevent power oversupply to the grid such that a grid voltage stays within an appropriate voltage range. Accordingly, in case of power oversupply, it is necessary to suppress the power supply by stopping a converter and an inverter of the power supply apparatus so as to prevent an increase in the grid voltage. When the power supply is suppressed, the grid voltage drops, thereby preventing the increase in the grid voltage exceeding the appropriate voltage range.
There is known a power supply apparatus that, in order to prevent the power oversupply to the grid, includes a voltage sensor for measuring the grid voltage and controls a function of the power supply apparatus based on the voltage measured by the voltage sensor (for example, see Patent Document 1).